The invention relates to automatic speech recognition utilizing a language model with language model values which are determined from a comparatively long training text, an adaptation of the language model being performed on the basis of notably short text segments in given cases, notably for given types of text.
For automatic speech recognition a series of test values is derived from a speech signal, said test values being compared with stored reference values, series of reference values representing words of a predetermined vocabulary. The comparison yields scores which are linked to language model values at word boundaries. These language model values represent the probabilities of given word sequences of limited length and have been determined from a large quantity of training speech signals. Consequently, long-term variations in usage, for example theme or style of speaking, cannot be taken into account by means of these language model values. Because suitably adapted language model values have a considerable effect on the reliability of the speech recognition, it is desirable to adapt the language model values as well as possible on the basis of a small quantity of current text. This adaptation can be performed during the recognition of a special text or also subsequent to the recognition of a part of the text after manual correction of recognition errors.
A method of forming an adaptive language model is disclosed in IEEE Trans. on Pattern Analysis and Machine Intelligence, Vol. 12, June 1990, pp. 570 to 583, and is known as the CACHE language model. According to this method relative frequencies N.sub.a (w)/N.sub.a are calculated, where N.sub.a (w) represents the frequency of the word w in the adaptation material and N.sub.a stands for the overall number of words in the adaptation material. The adaptive cache language model P.sub.a (w.vertline.h) is then calculated from the basic language model P.sub.s (w.vertline.h) as: ##EQU1## in which the interpolation parameter .lambda. is either fixed or determined for each adaptation in such a manner that the adaptation material is described as well as possible, as is known from Proc. ICASS, Minneapolis, USA, Vol. II, April 1993, pp. 585 to 593. The resultant adaptation of the language model values to an instantaneous text, however, still is not optimum.